Answer
$$ ax+by+cz=0.$$
Work Step by Step
The equation of any plane is given by
$$ a(x-x_{0})+b(y-y_{0})+c(z-z_{0})=0.$$
When the plane passes through the origin, then we put $(x_0,y_0,z_0)=(0,0,0)$
Hence the equation of any plane that passes through the origin takes the form
$$ ax+by+cz=0.$$
